1. Field of the Invention
This invention relates to a method of filtering a fluid.
2. Background of the Invention
Generally, when grinding and polishing or milling of inorganic or organic solids such as metals and ceramics is carried out, fine particles are generated. These fine particles are generally washed away by means of a fluid such as water and discharged as wastewater or sewage. This invention relates to a system for reusing industrial wastewater.
Reducing industrial waste is a serious current ecological theme now and is an important business issue for the 21st century. Among industrial waste, there are various kinds of wastewater and sewage.
In the following description, water or other fluid substances containing matters to be removed will be called wastewater. Such wastewater has removable matters extracted by an expensive filtration processing apparatus or the like; the wastewater filtered becomes clean and is reused. Wastewater containing unremovable matters is further processed or disposed as industrial waste. Filtered water may be reused or returned to nature.
However, because of high plant costs related to running a filtration process, employing these apparatuses poses a very difficult problem.
Also, high costs of sewage treatment are a serious problem, and therefore, a system with low initial cost and low running cost is urgently needed.
As an example, wastewater treatment in the semiconductor field will be described below. Generally, when grinding or polishing a metal, semiconductor or ceramic sheet, such factors as limiting temperature increases of equipment, improving lubrication, and preventing adhesion of waste produced by grinding and cutting the sheet are considered, and a fluid such as water is supplied to the sheet.
When semiconductor wafers are formed of ingot by grinding and slicing the ingot into wafers, or when a semiconductor wafer, which is a sheet of a semiconductor material, is diced or back-grinded, pure water is supplied. For preventing a temperature rise of a dicing blade in a dicing apparatus and for preventing dicing waste from adhering to the wafer, pure water is made to flow on the semiconductor wafer, or a nozzle for discharging water is mounted so that pure water will strike the blade and the wafer. Also, when the thickness of a wafer is reduced by back-grinding, pure water is supplied for similar reasons.
Wastewater containing semiconductor wafer grinding waste or polishing waste is filtered and thereby turned into clean water and returned to nature or reused, and concentrated wastewater is collected.
At present, there are two semiconductor manufacturing methods for processing wastewater containing mainly Si waste: a coagulating sedimentation method, and a method combining filtration and a centrifugal separator.
In the coagulating sedimentation method, PAC (poly-aluminum chloride) or A12 (SO4) 3 (band sulfate), for example, is mixed with wastewater as a coagulant. Si reactants are produced, and the wastewater is cleaned by removing those reactants.
In the method of combining filtration and centrifugal separation, wastewater is filtered, and the concentrated wastewater is put in a centrifugal separator and collected as sludge. Water obtained by filtering the wastewater is discharged into nature or reused.
As shown in FIG. 13, wastewater produced during dicing is collected in a raw water tank 201 and fed by a pump 202 to a filtering apparatus 203. Because the filtering apparatus 203 is fitted with ceramic or organic filters F, filtered water is fed through a pipe 204 to a water tank 205 and reused, or it is discharged to the environment.
Since the filters F gets clogged, the filtering apparatus 203 is periodically washed. This is accomplished by, for example, closing a valve B1 on the raw water tank 201, opening a valve B3 and a valve B2 for feeding washing water to the raw water tank 201, and back-washing the filters F with water from the water tank 205. Wastewater with highly concentrated Si waste is returned to the raw water tank 201. Concentrated water in a concentrated water tank 206 is transported through a pump 208 to a centrifugal separator 209, and is separated by the centrifugal separator 209 into sludge and liquid. The sludge containing Si is collected in a sludge collecting tank 210, and the liquid is collected in a liquid tank 211. Also, water of the liquid tank 211 in which separated liquid is collected is transported through a pump 212 to the raw water tank 201.
Similar methods to those above have also been employed in the collection of waste produced by grinding and polishing solids and sheets having metal materials such as Cu, Fe, Al as their main waste material and solids and sheets made of inorganic materials such as ceramics.
In the coagulating sedimentation method, a chemical as a coagulant is mixed with the filtered water. However it is very difficult to determine the necessary and sufficient amount of chemical that the Si waste will completely react with, and inevitably excess chemical is introduced and some chemical will remain unreacted. In contrast, if the amount of chemical is low, not all the Si waste will coagulate and some Si waste will remain in the solution.
When the amount of chemical is excessive, some chemical remain in the supernatant liquid. The supernatant liquid may not be reusable because of possible undesirable chemical reactions with the excess chemical. For example, filtered water with the excess chemical cannot be reused on a wafer during dicing, because it causes an undesirable chemical reaction.
Floc, a reactant of chemical and silicon waste, is produced as a suspended solid. For forming floc, the pH conditions are strict; pH of about 6 to 8 must be maintained using an agitator, a pH measuring apparatus, a coagulant pouring device and control devices. For example, for a wastewater processing capacity of 3 m3/hour, a tank with a diameter of 3 meters and a depth of 4 meters (a sedimentation tank of about 15 m3) would be necessary, and it becomes a large system requiring an installation area of about 11 metersxc3x9711.
Furthermore, some floc may not settle and drift out of the tank, making the collection difficult. Hence, there are problems such as a high initial cost of the filtration system because of the plant size, difficulties of reusing the filtered water, and a high running cost of the system because a chemical is used.
The reuse of water is possible with the method combining filtration and a centrifugal separator of 5 m3/hour because filters F (those made from polysulfone fiber, called UF modules, or ceramic filters) are used in the filtering apparatus 203. However, four filters F are installed in the filtering apparatus 203, and because the life of the filters F is about a year, it is necessary to replace the expensive (approximately 500,000 yen/unit) filter at least once a year. Furthermore, the load on the motor of the pump 202 for applying water to the filtering apparatus 203 is large because the filtration method is such that the filters F are of a pressurized type, and the pump 202 is a high-capacity type. Of the wastewater passing through the filters F, about ⅔ is returned to the raw water tank 201. Also, because wastewater containing silicon waste is transported by the pump 202, the internal walls of the pump 202 are scratched, cutting the life of the pump 202 short.
Hence, the cost of electricity for the motor is high and the running cost is also very high because there are replacement costs of the pump P and the filters F. FIG. 12 shows comparative data of the above system and a system of the invention described in the disclosure below. There are problems such as the size of the system, replacement of the filters, washing of the filters, and running costs.
To remove solid matter damaging to the earth""s environment as much as possible, various devices must be added, and therefore, the filtration system, must necessarily becomes large, leading to enormous initial costs and running costs.
The invention provides a simple filtering method by which clean water can be obtained effectively.
The invention can solve the above-mentioned problems by removing removables included in a fluid with a filter made from at least some of those removables.
When a filter is made with removables, it is possible to form filter pores even smaller than the removables constituting the filter. Therefore it is possible to extract still smaller removables by way of these small pores.
The invention can pass a fluid including removables through a first filter and form on the first filter surface a second filter consisting of the removables. The second filter can thereby remove other removables from the fluid.
On the first filter surface, a second filter made up of smaller pores than the pores of the first filter can be made to improve the filtering performance.
The invention can also recirculate a fluid including removables through a first filter to form on the first filter surface a second filter consisting of the removables.
As a result of recirculation, a second filter made up of smaller pores than the pores of the first filter grows on the first filter, and because small removables having passed through the first filter are also recirculated, the second filter is able to catch even smaller removables having passed through the pores of the first filter.
The first filter or the second filter can accommodate removables of different sizes.
The layered first and second filters with pores can pass fluid through and are able to catch small removables of different sizes in the wastewater.
The removables can have a particle diameter distribution having two peaks and the pores of the first filter can have a size between the two peaks.
With the pores of the first filter being between the two peaks, removables of the larger particle diameter are first trapped. Then as trapped removables are layered on the first filter in various arrangements, a second filter with smaller pores is formed. The second filter is then able to pass fluid and catch smaller removables.
The invention can detect with a detecting means the concentration of removables in the fluid passing through the first filter and can stop the recirculation when the concentration falls below a predetermined value.
A filtered fluid with mixtures of small removables can be recirculated to create a filter that will catch even those small removables. Therefore, by monitoring the fluid for a predetermined degree of concentration of removables as the fluid is recirculated, the fluid can be filtered to a target filtering accuracy.
The invention can further detect with a detecting means the concentration of removables in the fluid passing through the first filter, and can restart the recirculation when the concentration rises above a second predetermined value.
If the first filter breaks down or the second filter crumbles, the filtered water will contain removables that should have been filtered out, and this will create a problem during reuse. However, if a failure is detected, recirculation can be started immediately. This will prevent producing filtered water containing removables that should have been filtered.
The invention also can suck the fluid through the first filter.
For a sucking type, the reservoir tank of an open type can be used in which the fluid is stored and the filters are immersed. For a pressurized type, the reservoir tank must be sealed, and hence, this requires a complicated construction.
An external force may be applied to the second filter surface. Because the second filter consists of removables that have aggregated, if an external force is applied, it is possible to remove the whole of the second filter or a surface layer of the second filter to refresh and maintain the filtering performance.
The removables of the second filter surface can be desorbed with an external force. It is possible to desorb removables constituting a cause of clogging or forming gaps, and provide passages for fluids.
The first filter can be made of a polyolefin high polymer. The first filter can have resistance to alkalis and acids, and therefore, even fluids with mixed chemicals can be filtered. Also, coagulating sedimentation is possible with the first filter still immersed.
The removables can be inorganic solids or organic solids.
Each aspect of the method described below is separately illustrative of the various embodiments of the invention and is not intended to be restrictive of the broad invention.
A first aspect of the method is a method of filtering an object in a fluid, comprising the steps of:
preparing a filter by supplying the fluid including removables to a first filter and depositing the removables on the first filter surface so as to form a second filter; and
filtering the removables by supplying the fluid to the filter and thereby removing the removables from the fluid.
A second aspect of the method is a method of filtering an object in a fluid according to the first aspect, wherein the step of preparing a filter comprises a steps of supplying the fluid including removables to the first filter, and circulating it.
A third aspect of the method is a method of filtering an object in a fluid according to the first aspect, wherein the second filter comprises removables of different sizes.
A fourth aspect of the method is a method of filtering an object in a fluid according to the first aspect, wherein the removables comprise different sizes particles, and said second filter is made up of pores which are larger than the smallest sizes of the particles and larger than the largest sizes of the particles.
A fifth aspect of the method is a method of filtering an object in a fluid according to the fourth aspect, wherein the fluid including removables comprise waste water exhausted in a step of forming a semiconductor wafer of semiconductor ingot or a step of dicing the semiconductor wafer.
A sixth aspect of the method is a method of filtering an object in a fluid according to the first aspect, wherein the removables have a particle diameter distribution having two peaks and the pores of the first filter being of a size between the two peaks.
A seventh aspect of the method is a method of filtering an object in a fluid according to the first aspect, wherein an amount of the removables having a size larger than pores of the first filter is more than that of the removables having a size smaller than pores of the first filter.
An eighth aspect of the method is a method of filtering an object in a fluid according to the first aspect, wherein said step of filtering comprises a step of circulating the fluid for a constant time since starting removing.
A ninth aspect of the method is a method of filtering an object in a fluid according to the eighth aspect, wherein said step of circulating comprises a step of detecting an inclusion degree of removables included in the fluid passing through the filter, and stopping circulation of the fluid when the detected degree has fallen below a first constant value.
A tenth aspect of the method is a method of filtering an object in a fluid according to the ninth aspect, wherein said step of circulating comprises a step of detecting an inclusion degree of removables included in the fluid passing through the filter, and starting circulation of the fluid again at the time point when the detected degree has exceeded a second constant value.
An eleventh aspect of the method is a method of filtering an object in a fluid according to the ninth aspect, wherein said step of detecting comprises a step of detecting a transparence of the fluid by light sensor.
A twelfth aspect of the method is a method of filtering an object in a fluid according to the first aspect, wherein said step of filtering comprises a step of filtering the fluid while sucking through the filter.
A thirteenth aspect of the method is a method of filtering an object in a fluid according to the twelfth aspect, wherein an applied suction pressure in sucking is within a range of 0.2 to 0.5 kg/cm2.
A fourteenth aspect of the method is a method of filtering an object in a fluid according to the first aspect, further comprising a step of applying an external force to a surface of the filter so that a constituent of the second filter can be moved.
A fifteenth aspect of the method is a method of filtering an object in a fluid according to the fourteenth aspect, wherein said step of applying an external force comprises a step of applying the external force intermittently.
A sixteenth aspect of the method is a method of filtering an object in a fluid according to the fourteenth aspect, wherein said step of applying an external force comprises a step of applying gas flow along a surface of the first filter.
A seventeenth aspect of the method is a method of filtering an object in a fluid according to the fourteenth aspect, wherein said step of applying an external force comprises a step of applying a force so as to make a part of the constituent of the second filter released.
An eighteenth aspect of the method is a method of filtering an object in a fluid according to the fourteenth aspect, wherein said step of applying an external force comprises a step of controlling a force so that a thickness of the second filter is constant.
A nineteenth aspect of the method is a method of filtering an object in a fluid according to the fourteenth aspect, wherein said filter is disposed in perpendicular direction and said external force comprises a raising force of a bubble.
A twentieth aspect of the method is a method of filtering an object in a fluid according to the fourteenth aspect, wherein said step of applying an external force comprises a step of applying a mechanical vibration.
A twenty first aspect of the method is a method of filtering an object in a fluid according to the fourteenth aspect, wherein said step of applying an external force comprises a step of generating a sonic wave.
A twenty second aspect of the method is a method of filtering an object in a fluid according to the fourteenth aspect, wherein said step of applying an external force comprises a step of generating a flow of the fluid.
A twenty third aspect of the method is a method of filtering an object in a fluid according to the first aspect, wherein said first filter is made of polyolefin high polymer.
A twenty fourth aspect of the method is a method of filtering an object in a fluid according to the first aspect, wherein said first filter has an uneven surface.
A twenty fifth aspect of the method is a method of filtering an object in a fluid according to the first aspect, wherein said first filter has a bag typed filter in which clearance is formed and in which suction pipe for sucking is inserted.
A twenty sixth aspect of the method is a method of filtering an object in a fluid according to the first aspect, wherein said second filter comprises at least one element of Si, SiGe, Al2O3, Si oxide, metal oxide, and IIa-VIIa, IIb-VIIb group of elements.
A twenty seventh aspect of the method is a method of filtering an object in a fluid according to the first aspect, wherein said second filter comprises Si.
A twenty eighth aspect of the method is a method of filtering an object in a fluid according to the twenty seventh aspect, wherein said second filter comprises mainly flake type of Si.
A twenty ninth aspect of the method is a method of filtering an object in a fluid according to the first aspect, wherein said second filter comprises a mechanical processing waste generated in the mechanical processing step.
A thirtieth aspect of the method is a method of filtering an object in a fluid according to the twenty ninth aspect, wherein said mechanical processing step comprises a polishing step or grinding step.
A thirty first aspect of the method is a method of filtering an object in a fluid according to the first aspect, wherein said mechanical processing waste is a waste of dicing.
A thirty second aspect of the method is a method of filtering an object in a fluid according to the thirty first aspect, wherein said step of preparing a filter comprises a step of adding a flake shaped material in to the fluid.
Removables may comprise many kinds of form, such as flake typed Si. Hence, a good filter can be obtained without blocking.
If the removables are solids, small gaps of various shapes can be formed by the particles of differing diameter sizes. Consequently, smaller removables can be trapped and also more passages for fluid can be provided.